(1). Field of the Invention
The invention relates to piezoelectric actuators for rotary ultrasonic and linear motors. These can be employed for driving motors for linear and rotary motions. Moreover, they can be used for driving instrument racks, optical lenses, optical write and read heads as well as other similar devices requiring a high positioning accuracy, small dimensions of the drive system and low supply voltages.
(2). Description of Related Art
Known are piezoelectric actuators for ultrasonic motors comprised of a metallic resonator for acoustic waves, with pasted-on lamellar piezoelectric elements, which excite acoustic waves in the resonator (see, inter alia, U.S. Pat. Nos. 5,665,918; 5,672,930).
The drawback of these actuators resides in the large dimensions thereof, which are defined by the dimensions of the metallic resonator. Moreover, these actuators have a small volume of the piezoelectric active part as well as a great thickness of the piezoelectric element, which reduces the electromechanical coupling coefficient significantly so that the excitation voltage is considerably higher. Such actuators additionally have a complicated structure, which requires an assembly by hand and increases the manufacturing costs.
Further known are piezoelectric actuators for ultrasonic motors (see, inter alia, U.S. Pat. Nos. 5,134,334; 5,416,375) whereof the piezoelectric elements are connected as thin lamellae packets to the metallic resonator. The drawbacks of these actuators reside in their large dimensions, their complicated structure and the high manufacturing costs.
Additionally known are piezoelectric actuators for ultrasonic motors comprised of piezoelectric plates, with electrodes mounted on the two main surfaces thereof (see, inter alia, U.S. Pat. Nos. 6,384,515; 7,714,833).
The drawback is that these actuators require a high excitation voltage. This is due to the fact that a bending resonance with a low electromechanical coupling coefficient is used. The maximum excitation voltage of such actuators reaches values of 300 Veff. Another significant disadvantage is the presence of a second longitudinal resonance which is within the range of the bending resonance. This makes it harder to control these actuators, with the result that it is impossible to construct simple excitation circuits operated according to the self-excitation principle, whereof the excitation frequency is predetermined by the actuator itself. This leads to a significant reduction of the thermal stability of the drive, reduces the operational safety of the actuators and renders the production more expensive.
Further known are miniaturized piezoelectric actuators for ultrasonic motors, in which the multilayer piezoelectric element is pressed into the body of the metallic resonator (see, inter alia, US 2004/0256954 A1). These actuators are small in size and have low excitation voltages.
The operating principle of these actuators is based on the excitation of two types of standing waves in the resonator on resonance frequencies lying close together. Therefore, to reverse the motion of the mobile element, the frequency of the excitation circuit has to be tuned from the one to the other resonance frequency. This does not allow the configuration of an excitation circuit based on the self-excitation principle, the frequency of which is predetermined by the mechanical parameters of the actuator. By this, the operating stability of the actuator is reduced, the structure of the excitation circuit is rendered more complicated and the drive system as a whole is rendered more expensive.